1. Field of the Invention
The present invention relates to well bore fluids and, more particularly, to drilling fluids or muds of the oil based variety, so called "invert" emulsion fluids.
2. Description of the Background
Drilling fluids or muds are based on aqueous formulations or oil-based formulations, the latter being sometimes referred to as invert emulsion muds. Invert emulsion drilling fluids find particular application where the use of water-based fluids would result in damage to the formation through which the drilling is progressing. For example, it is known that certain types of shale will heave and collapse if water-based drilling fluids are used. Since the oil-based drilling fluids do not result in any swelling of the shale, their use circumvents the heaving problem. Invert emulsion muds basically contain an oleaginous medium, such as a hydrocarbon liquid as the continuous phase, water as the dispersed phase, various emulsifying agents, wetting agents, weighting agents and viscosifiers, such as amine treated clays.
Water-based and most oil-based drilling muds are relatively easy to effectively viscosify. However, oil-based drilling fluids which use low aromatic content hydrocarbon liquids as the oleaginous phase are more difficult to viscosify using prior art gellants, such as amine treated clays. The use of oil-based drilling fluids having low toxicity, i.e. low aromatic content, is desirable and in many cases mandatory where the drilling conditions involve possible ecological damage, e.g. offshore drilling.
U.S. Pat. Nos. 4,425,462 and 4,442,011 disclose oil-based drilling fluids which employ sulfonated ethylene/propylene/5-phenyl-2-norbornene terpolymers (EPDM polymer). The use of organophilic clays as oil-based mud gellants is disclosed, for example, in U.S. Pat. Nos. 4,287,086 and 4,105,578. A prior art publication entitled TekMul 1949 discloses that EPDM polymers can be used as a replacement for all or part of the organophilic clay requirements of an oil-based mud. However, the publication does not suggest that a synergistic effect on viscosity is achieved by the use of the clay and the EPDM polymer, particularly in low aromatic content liquids.